In a typical construction machine such as hydraulic excavators and crawler cranes, a revolving frame is mounted on lower machinery through a turning mechanism, the lower machinery having a traveling function. A work equipment and a cab are mounted on the revolving frame, so that the work equipment can be operated to carry out excavation, lifting or the like while the revolving frame turning in accordance with working conditions. In this arrangement, the cab is installed in a position adjacent to the base of the revolving frame at which the work equipment is mounted, for effectively performing operation.
The above construction machines and, particularly, hydraulic excavators are not always operated on plain lands but frequently operated on rough terrains such as unleveled lands and slopes. When a hydraulic excavator is operated in such an unstable condition, it sometimes happens that an unexpected external force or erroneous operation causes the hydraulic excavator to overturn and the cab is deformed owing to shocks occurring at the time of the overturn. Therefore, there have been demands for a structure for protecting the operator sitting on the operator's seat of the cab from an injury in the event of an overturn. Apart from cab protecting guards provided on the outer side of a cab, there have been proposed measures of reinforcing a whole cab to meet the demands. One example of the measures is disclosed in Patent Document 1. The protective structure of Patent Document 1 is formed such that a vertical stiffening material is inserted into a hollow part between an inner panel and outer panel which are the chief components of the cab, thereby preventing deformation caused by an overturning load.
Another structure has been proposed in Patent Document 2 according to which a structural member is attached which is locally elastically deformable if a load is imposed on a framework serving as an essential element of the vehicle in the event of a crash, and the load caused by a shock is absorbed by the deformation of the structural member to protect the operator.
[Patent Document 1]
Japanese Unexamined Patent Application Publication No. 11-166247
[Patent Document 2]
Japanese Unexamined Patent Application Publication No. 2001-260773